1. Field of the Invention
The present invention relates in general to a method for forming a back-drilled plated through hole in a printed circuit board and the resulting printed circuit board.
2. Description of Related Art
Designers and manufacturers of printed circuit boards are constantly trying to develop new and better methods to make printed circuit boards. For instance, the designers and manufacturers have been trying to develop a new and better method to form back-drilled plated through holes in printed circuit boards. A traditional method for forming back-drilled through holes in a printed circuit board is described in detail below with respect to FIGS. 1A-1D.
Referring to FIGS. 1A-1D, there are shown partial cross-sectional side views of a traditional printed circuit board 100 at different stages during the formation of a back-drilled plated through hole 102. The exemplary multi-layered printed circuit board 100 shown has six stacked material insulator layers (typically laminates and prepregs) 104a, 104b . . . 104f separating five internal layers 105a, 105b . . . 105e and two external layers 108a and 108b. Internal layers 105a, 105b and 105e are signal layers. And, internal layers 105c and 105d are plane layers.
In the first step to form the back-drilled plated through hole 102 in printed circuit board 100, a pilot hole 106 (e.g., first hole 106) having a diameter d1 is drilled through the printed circuit board 100 (see FIG. 1A). In an illustrative example, the thickness of the printed circuit board 100 is 0.23″ and the diameter d1 of the pilot hole 106 is 0.030-0.032″. Secondly, the walls of the pilot hole 106 and predetermined areas 108a and 108b on the surfaces 110a and 110b of the printed circuit board 100 are plated with a thin layer of conductive material 112 (e.g., copper) (see FIG. 1B). In the illustrative example, the diameter d2 of the plated pilot hole 104 is 0.026″. Thirdly, a drill 114 having diameter d3 is used to back-drill concentrically around and through a predetermined depth of the plated pilot hole 106 (see FIG. 1C). In the illustrative example, the drill 114 has a diameter d3 in the range of 0.040-0.045″ and is shown going through stacked layers 104d, 104e and 104f. Fourthly, the drill 114 is removed from the printed circuit board 100 which now has a via 116 formed therein that includes the plated through hole 102 and a back-drilled hole 118 (see FIG. 1D).
The traditional method for forming the back-drilled plated through hole 102 in the printed circuit board 100 has a major drawback because of a registration tolerance requirement that requires the back-drilled hole 118 to have a diameter d3 that is much larger than the diameter d1 of the pilot hole 106. In particular, the registration tolerance requirement demands that the drill 114 used to make the back-drilled hole 118 have a diameter d3 that is at least 0.010″ larger than a diameter of the drill (not shown) used to make the pilot hole 106. This means that the diameter of the back-drilled hole 118 is a lot larger than the diameter of the plated through hole 102 which negatively impacts the clearance requirements for signal trace(s) 105e and ground plane(s) 105d in the back-drilled layers and associated material insulator layers 104d, 104e and 104f. Accordingly, there is a need for a new way to form a back-drilled pilot through hole in a printed circuit board that overcomes the aforementioned shortcoming of the traditional back-drilled method. This need and other needs are satisfied by the method and printed circuit board of the present invention.